IPL+UBI: Flexible and Reliable with Linux as the BootloaderIPL+UBI: Flexible and Reliable with Linux...

IPL+UBI: Flexible and Reliable with Linux as the BootloaderCELF Embedded Linux Conference Europe 2010

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John OgnessLinutronix GmbH

IPL+UBI: Flexible and Reliablewith Linux as the Bootloader


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Embedded Board Trends

• MMC / SD• CompactFlash• SATA• USB (OTG)• Firewire• RFID• Ethernet (wireless)

• TFT LCD• Touchscreen• Audio• Sensors

‣ temperature‣ brightness‣ proximity‣ accelerometer


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Linux DriversLinux DriversNecessaryNecessary


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... but what about... but what aboutthe bootloader?the bootloader?


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Bootloader Responsiblities

• init low-level hardware (RAM, MTD)• load/run kernel from MTD


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• init low-level hardware (RAM, MTD)• load/run kernel from MTD‣ failsafe boot environment


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⁃ show a splash screen⁃ play startup music⁃ BIOS-like application (user interaction)⁃ load/run kernel from external device (USB / MMC / SATA / ethernet)

‣ failsafe boot environment


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Do we really want toimplement all these driversagain for the bootloader?


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Yes!

Yes! Yes!

NO!

Original icons (Restroom_symbols.svg) copyright Amy Youngunder the Creative Commons Attribution-ShareAlike 3.0 License.


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Drivers in Bootloaders

Advantage• early access to hardware


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Drivers in Bootloaders

Advantage• early access to hardware

Disadvantages• less availability• less tested• less maintained• often incomplete• less support for out-of-tree drivers


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What if we use LinuxWhat if we use Linuxas the bootloader?as the bootloader?


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Linux as Bootloader

• use a minimal kernel (to reduce startup times)

• load drivers on demand as needed

• BIOS-like application runs in a Linux userspace environment

• load final kernel from wherever

• use kexec to run final kernel


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... but how does the Linux... but how does the Linuxbootloader get loaded?bootloader get loaded?


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IPL (Initial Program Load)

• also known as “X-Loader” or “Stage One Bootloader”

• less than 16KB in size

• performs minimal tasks required to start the bootloader


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IPL


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LinuxBootloader


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Boot Procedure

1. boot ROM loads and runs IPL

2. IPL loads its configuration from MTD

3. IPL loads and runs minimal kernel from MTD

4. BIOS-like application runs (initramfs)

5. load and run final kernel from wherever


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Boot Procedure







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NAND (MTD)


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NAND (MTD)• data organized in addressable pages

‣ 512, 2048, or 4096 bytes per page (+ spare bytes for ECC)

• pages organized in erase blocks‣ 32, 64, or 128 pages per erase block

• individual pages may be programmed

• to reprogram a page, the entire erase block must first be erased

• limited program-erase cycles


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NAND (effective use)• wear-leveling

‣ wear down the entire MTD evenly

• bad block management‣ NAND erase blocks will go bad

• error correction code‣ bit-flips when reading NAND will occur

• data scrubbing ‣ keep data on “healthy” erase blocks


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UBI (Unsorted Block Images)• volume management system for raw flash devices

• maps logical erase blocks to physical erase blocks

• transparently implements effective MTD usage

• additional data integrity features ‣ erase block checksums ‣ erase block sequence numbers


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UBI (a few nice features)• supports multiple volumes within a single MTD partition

• supports static volumes ‣ useful for storing data blobs (e.g. kernel)

• supports dynamic volumes ‣ used for filesystems (e.g. UBIFS)

• individual volumes may be updated ‣ ... with effective MTD usage


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UBI (how it works)• UBI management “attaches” to a specified MTD partition

• UBI scans the MTD partition ‣ LEB/PEB mapping created in RAM ‣ erase block checksums and sequence

numbers used to identify valid data ‣ “global” sequence number set to the

highest valid erase block sequence number found (in RAM)


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UBI (in Linux)• device nodes are created for each UBI volume ‣ static volumes can be read ‣ dynamic volumes can be mounted

• userspace UBI tools available ‣ ubiformat ‣ ubiattach / ubidetach ‣ ubinfo ‣ ubinize / ubimkvol / ubirmvol ‣ ubiupdatevol


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Boot Procedure







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Boot Procedure


2. IPL loads its configuration from an UBI static volume (MTD)

3. IPL loads and runs minimal kernel from an UBI static volume (MTD)




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IPL+UBI (example)• 1st MTD partition = 4 erase blocks ‣ each has a copy of the IPL

• 2nd MTD partition = 10 MB, UBI managed with (at least?) 4 static UBI volumes ‣ IPL config ‣ minimal kernel ‣ production kernel ‣ failsafe kernel

• 3rd MTD partition = “the rest” ‣ most likely also UBI managed ;-)


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IPL+UBI (example cont.)• OMAP3530 (ARM Cortex A8 @ 720 MHz)

• 64 MB NAND (0.5 KB pages, 32 PPB)

• IPL size = 12 KB ‣ includes simple UBI library

• IPL config size = 0.1 KB

• “minimal” kernel size = 940 KB ‣ compressed using LZMA ‣ includes drivers for NAND, UBI, display

‣ includes initramfs with busybox and kexec


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IPL+UBI (example cont.)Startup time until Tux visible:

• UBI scan (2nd MTD partition) = 180 ms

• load IPL config = 2 ms

• load minimal kernel = 700 ms

• kernel startup (until Tux) ≈ 3000 ms ‣ decompress kernel = 1000 ms ‣ boot until Tux visible ≈ 2000 ms

Total time to Tux ≈ 4 seconds


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Total Time to Tux

IPL+UBI882 ms

Linux Startup~3000 ms


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Questions / CommentsRCPT TO:<[email protected]>

Other Presentations On This Topic09:45 - The Right Approach to Minimal Boot Times

11:00 - Barebox: Booting Linux Fast and Fancy

15:20 - Flash Filesystem Benchmarks

16:10 - YAFFS Updates

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